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One day last spring Boston attorney Ben Seisler, 33, told his friend Vas Leckas he wanted to see him. He said had an important personal secret he needed to share.

Leckas’s first thought was it had something to do with Seisler’s upcoming wedding. His second thought (“I have an active imagination,’’ he says.) was that Seisler, whose parents live in Belgium, had inherited a valuable Flemish master painting. “I would have suggested he auction it off, with my assistance,’’ said Leckas, 32, a software salesman. “With a hefty art handler fee for me.’’

His fantasies could not have been more off base. When they met, Seisler confided that he’d been a sperm donor for three years while in law school, and recently learned he was the biological father of a “little dude,’’ who wanted to meet him. Also, he confessed, a little girl.

And one more thing, Seisler said. They have, “like, 70 or so siblings.’’

Since that conversation Seisler has learned about even more children conceived with his sperm, and he is pretty sure that number will go up. He is registered on an online registry called the Donor Sibling Registry that matches children conceived by sperm donors with their biological fathers and half-siblings. Based on his calculations, “I have reason to expect between 120 and 140,’’ said Seisler. He recently met two of them - a 7-year-old girl and her 4-year-old brother, who bore what Seisler calls “a bizarre resemblance’’ to him - as part of a reality documentary special, “Style Exposed: Sperm Donor,’’ which airs Sept. 27 on the Style network.

His experience puts a new spin on the definition of extended family, and makes for entertaining TV. “I don’t want to sound cold and calculating,’’ Leckas told Seisler after hearing the news, “but you can’t be at 70 birthday parties a year. There’s only so much Chuck E. Cheese you can take.’’

The situation highlights the complicated issues that are starting to emerge now that a sizable cohort of donor-conceived children are growing up and wanting information about their biological fathers. Issues such as: What if the kids want to meet the fathers? What if children from the same sperm donor meet each other, and unwittingly get married? When’s the right time to tell your girlfriend that you were a sperm donor? (Seisler broke the news to his on their third date.)

These are issues a lot of men may not have considered back at the sperm bank, a lucrative source of quick cash. Seisler averaged $150 per donation and said the transaction seemed pretty uncomplicated.

“They told me I’d be anonymous,’’ said Seisler, who donated sperm at the Fairfax Cryobank in Virginia to help defray his law school bills from George Mason University. “That made sense to me. I really didn’t think too much about people trying to find me.’’

It made sense to him at the time because it was a very different time. A decade ago, it was hard to anticipate that offspring conceived with his sperm would grow up in a culture that valued such openness about their origins. Or that there would be an online sperm donor registry to help donor children find their biological family members. (To say nothing of reality TV, to record dramatic reunions in real time.)

“The Web was there, but not like today. And no ‘23andMe,’ ’’ said Seisler, referring to a retail DNA testing service. “And I hadn’t heard of the Donor Sibling Registry.’’

The registry was created in 2000 by Wendy Kramer, a Colorado single mother whose son, conceived with the help of a sperm donor, was hungry for information about his heritage. “When he was 2 he came to me and said, ‘So did my dad die, or what?’ ’’ Kramer said. “By the time he was 6, he was saying pretty adamantly, ‘I want to know who my biological father is.’ As he got older, he started to wonder if he had any half-siblings. His thinking was there are other half brothers and sisters out there, and if we could all meet each other, we could see the invisible side of ourselves in each other.’’

She tried to locate them, but neither the sperm bank nor the medical clinic would help make the connection. So she took matters into her own hands, and started a Yahoo group for donor families looking for information about their biological relatives. (Eventually, her son did meet his father, and has met three of his six half-sisters.)

The Yahoo group grew into a nonprofit Web-based registry: Parents or children who register can type in their sperm donor’s cryobank identification number and connect with their half-siblings, if they, too, have registered. Donors can also post their information, which enables people to send them anonymous messages via the website.

That’s what Seisler did in 2005 after reading a newspaper article about the registry. Apprehensive but curious, he checked out the site. He remembered his donor number - 2149 -and posted it anonymously.

Within a week, there was an e-mail in his inbox from a parent who’d used his sperm to have a child. Soon there were more e-mails: Ten. Twenty. Thirty. He keeps track of them all on an Excel spreadsheet.

“I know of 75 kids,’’ said Seisler, who married last month and just moved to Albany. “The messages turned out to be very consistent. People thanked me and asked basic questions about medical information.’’ A few wanted to know if he’d be willing to be contacted by their kids when they got older.

He said he’d be happy to be contacted. “It was almost shocking at first [to hear from families], but at the same time, you realize that . . . what happened facilitated people having families. It was heartwarming.’’

One of the parents he heard from was a Seattle single mother named Sharon. She used Seisler’s sperm to have two children, selecting him from an online cryogenic bank catalog because his profile appealed to her. “He met the religion criterion. He met the looks criterion,’’ said Sharon, who asked that her last name not be used to protect her children’s privacy. “I wanted someone who looked enough like me that people would never question that they were mine. I wanted a professional. I wanted someone athletic.’’

Seisler was her man. “I’d read that finding the right donor is kind of like finding your soul mate,’’ she said. “When you know, you know. And that’s what happened.’’

One day about four years ago, she logged on to the Donor Sibling Registry and was surprised to see that Seisler had registered as a sperm donor. “I was like, wow, why don’t I just send him an e-mail and say thank you,’’ she said. “I also wanted to see if sometime in the future he’d be open to answering questions or even knowing about the children. He wrote back very promptly and said he was interested. I thought he was as nice a guy as he came across in his profile.’’

She didn’t immediately reach out to him again, though she did join a chat group of some other mothers whose children’s biological father is Donor 2149. The oldest is 9, the youngest is an infant. “We are still having babies,’’ Sharon said. “We had our newest addition last month.’’

Last fall, she was contacted by Wendy Kramer asking if she’d be interested in being part of a reality TV documentary. The Style network would fly her family - including her parents - to Boston to meet Seisler.

She was hesitant at first, but agreed. “I didn’t want to have to kick myself, to say I had the chance and I blew it,’’ she said. “At least they can say they met their biological father and he is a decent guy.’’

They met in May to spend an afternoon at a miniature golf course in Marlborough, and if the reality TV show reflects reality, it’s clear that this new uncharted sperm donor world can be complex. In the show, as they prepare for their wedding, Seisler’s fiancée is clearly miffed about his situation, especially when he tells her on camera that the number is up to 70. She can barely bring herself to refer to them as “children.’’ (She prefers “offspring.’’)

“What if they all come knocking?’’ she asks Seisler, angrily. “I kind of deem it selfish. Did you think of the consequences that would come out of this?’’

Back in Seattle, Sharon struggles for a way to tell her daughter about Seisler.

“How did mommy get the sperm?’’ she asks Abby, who is 7.

“Google?’’ Abby replies.

The actual meeting is awkward at first, but turns poignant. “I don’t want Ben to leave,’’ Abby pleads at the end of the day. Sharon’s mother hugs Seisler and thanks him for her grandchildren. “If I had to pick a father for them, I couldn’t have picked anyone more perfect,’’ she said.

Seisler said he’s glad he met the kids but admits the experience was at times a bit surreal. “It was kind of wild. On the one hand, these kids are biologically my kids. On the other hand they are not my kids. I didn’t raise them. I have no control over how they are raised.

“There is no road map for this, no protocol to follow,’’ Seisler reflected. “This really is uncharted territory.’’